Just to clarify, a "black company" does not need to have any "appearance" of being legal or not. It is classified as so so long as it does not follow the laws or does not treat its employees with respect. It has been associated with the term "sweatshop" though the Japanese phrase does not necessarily refer just to factory-based labour. From the Japanese Wikipedia page, Konna Haruki defines it as "a growing company in a new industry that employs many young people and overwork them to the bone with either hard or illegal labour or power harassment and cause many employees to leave the job". Another definition describes it as "a company that acknowledges its abhorrent abuse of its employees' human rights but makes no effort to correct it". (see https://ja.wikipedia.org/wiki/%E3%83%96%E3%83%A9%E3%83%83%E3%82%AF%E4%BC%81%E6%A5%AD)
I actually planned on translating the list of points that identify signs of a "black company" but the list is far longer than I thought. The problem is without a doubt still rampart in society but the more awareness there is, the more room for improvement can be made.
Incidentally, those who are working in states with a non-liveable "minimum wage" should really help others out by raising their voices. A company that cannot turn a profit without having a turnover in staff is a company that should not even be existing.